IEはHTTP-EQHIV互換モードでスキップされていますか？

Answer 1

The article specifies the compatibility modes IE (8-9) can run in. IE5(.5) equals to quirks mode. IE8 cannot run in IE6 compatibility mode. IE6 does not absolutely render the same as IE5(.5) (unless it's in quirks mode).

If you want a reliable way to test in IE6, IE7, etc., you should try something like IETester (which I found to be reliable), Spoon, etc. if you don't want to use VMs.

You should also check this SO question. As stated there:

Also, don't trust IE8 compatibility mode.